Structural, Electronic and Magnetic Properties of X2yz Heusler Compounds Mn2vz (Z = Al, in), Co2yin (Y = V, Nb, Ti) and Xyz Half Heusler Compounds Coyin (Y= V, Nb, Ti) for Spintronic Application

Abstract

Heusler compounds have been predicted to be half-metallic possessing integer magnetic moments due to complete spin polarisation at the Fermi level, a property highly desirable in spintronics. However, there are yet to be any reports of 100% spin polarisation experimentally. In order to realise this predicted property and to avail these materials as a viable spin source, their structural properties need to be optimised. In this study, the structural, electronic and magnetic properties of selected Manganese (Mn) and Cobalt (Co) Heusler compounds have been investigated using Density Functional Theory (DFT) as implemented in the Vienna ab Initio Simulation package (VASP). In addition, polycrystalline samples were prepared by arc melting and their structural and magnetic properties were studied using magnetometry and electron microscopy techniques. The optimised lattice constants for Mn2VAl is 5.800Å while those of Mn2VIn , Co2VIn and Co2NbIn are 6.250Å , 6.000Å and 6.200 Å, respectively, with band structures having small band gap energies in one of the spin channels of approximately 0.2 eV and integer magnetic moments of 2 μB as predicted by the Slater-Pauling rule. While full-Heuslers Mn2VAl, Co2VIn and Co2NbIn and half Heusler CoVIn are predicted to be half-metallic at the optimised lattice constants, Mn2VIn is half-metallic at a reduced lattice constant of 6.000Å and displays additional properties such as tetragonal distortion and perpendicular magnetic anisotropy energy of 96.800 meV when doped with Co as well as 2.090meV - 6.790 meV for angström and nanometer range thickness Mn2VIn films. While Co2TiIn has a pseudo gap in the minority spin channel, the half-Heuslers CoTiIn and CoNbIn are found to be metallic. Single phase L21 structure for Mn2VAl and multiple cubic and tetragonal phases in Co2VIn and Mn2VIn were detected in the arc melted samples by XRD using Cu-Kα radiation and quantitative analysis and phase separation done by Rietveld refinement using the FullProf software. The lattice parameters for Mn2VAl, Co2VIn and Mn2VIn are 5.900 Å, 5.890Å and 5.830 Å respectively, which are comparable to the optimised values especially for Mn2VAl and Co2VIn. Composition and homogeneity were determined using Energy dispersive X-ray (EDX) spectroscopy and the scanning electron microscope (SEM), in which non-stoichiometric compounds Mn1.79VAl1.14, Co1.8VIn0.1 and Mn2.29V0.6In were found due to In melting separately and V segregation in the In based compounds. Mn1.79VAl1.14 was found to have a coercivity of 5.370 x 10-3 T and a saturation magnetisation of 1.740 μB/f.u, while the coercivity of Co1.8VIn0.1 is 3.400 x 10-2 T and magnetisation of 5.767 x 10-4 μB/f.u. Mn-V-In displayed martensitic and superparamagnetic characteristics with a blocking temperature of 40 K, a remanent magnetisation of 0.500 μB/f.u. at 2K and a coercive field of 4.678 x 10-1T. The versatility of Heusler compounds has been displayed in this work with the compounds showing from ab initio investigations, highly relevant properties to technology such as half-metallic near antiferromagnetism from Co doping, superparamagnetism, martensitic transformation and volume derived magnetic anisotropy in Mn2VIn, while in Co2YIn (Y = V, Nb, Ti), half-metallicity has been displayed.